Semiconductor device and method for fabricating the same

ABSTRACT

To provide a highly reliable semiconductor device exhibiting stable electrical characteristics. To fabricate a highly reliable semiconductor device. Included are an oxide semiconductor stack in which a first oxide semiconductor layer, a second oxide semiconductor layer, and a third oxide semiconductor layer are stacked, a source and a drain electrode layers contacting the oxide semiconductor stack, a gate electrode layer overlapping with the oxide semiconductor layer with a gate insulating layer provided therebetween, and a first and a second oxide insulating layers between which the oxide semiconductor stack is sandwiched. The first to the third oxide semiconductor layers each contain indium, gallium, and zinc. The proportion of indium in the second oxide semiconductor layer is higher than that in each of the first and the third oxide semiconductor layers. The first oxide semiconductor layer is amorphous. The second and the third oxide semiconductor layers each have a crystalline structure.

TECHNICAL FIELD

The present invention relates to a semiconductor device and a method forfabricating the semiconductor device.

Note that a semiconductor device in this specification refers to alldevices which can function by utilizing semiconductor characteristics,and electro-optical devices, semiconductor circuits, and electronicdevices are all semiconductor devices.

BACKGROUND ART

Attention has been focused on a technique for forming a transistorincluding a semiconductor thin film formed over a substrate having aninsulating surface (also referred to as a thin film transistor). Thetransistor is applied to a wide range of electronic devices such as anintegrated circuit (IC) or an image display device (display device). Asilicon-based semiconductor material is widely known as a material for asemiconductor thin film applicable to a transistor. As another material,an oxide semiconductor has been attracting attention.

For example, a transistor including an oxide semiconductor containingindium (In), gallium (Ga), and zinc (Zn) is disclosed in Patent Document1.

An oxide semiconductor film can be formed by a technique for forming athin film, such as a sputtering method. Further, the oxide semiconductorfilm can be formed at a relatively low temperature compared to a siliconsemiconductor or the like. Hence, the oxide semiconductor film can beformed to overlap with another transistor. For example, Patent Document2 discloses a semiconductor device in which a cell area is reduced byproviding, over a transistor including silicon, a transistor includingan oxide semiconductor layer serving as a channel formation region.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165528-   [Patent Document 2] Japanese Published Patent Application No.    2012-15500

DISCLOSURE OF INVENTION

In a transistor including an oxide semiconductor layer serving as achannel formation region, an oxygen vacancy (an oxygen defect) isgenerated by release of oxygen from the oxide semiconductor layer, and acarrier is generated due to the oxygen vacancy. Further, entry of animpurity such as hydrogen into the oxide semiconductor layer causesgeneration of a carrier.

The carrier generated in the oxide semiconductor layer increases theoff-state current of the transistor and a variation in thresholdvoltage. Thus, a shift in electrical characteristics of the transistoroccurs, leading to a reduction in reliability of a semiconductor device.

In view of the foregoing problem, an object of one embodiment of thepresent invention is to provide a highly reliable semiconductor deviceexhibiting stable electrical characteristics. Another object of oneembodiment of the present invention is to fabricate a highly reliablesemiconductor device.

A semiconductor device of one embodiment of the present inventionincludes an oxide semiconductor stack in which a first oxidesemiconductor layer, a second oxide semiconductor layer, and a thirdoxide semiconductor layer are stacked, and a first oxide insulatinglayer and a second oxide insulating layer between which the oxidesemiconductor stack is provided. In the semiconductor device, the firstoxide semiconductor layer, the second oxide semiconductor layer, and thethird oxide semiconductor layer each contain at least indium, theproportion of indium in the second oxide semiconductor layer is higherthan that in each of the first oxide semiconductor layer and the thirdoxide semiconductor layer, the first oxide semiconductor layer isamorphous, and the second oxide semiconductor layer and the third oxidesemiconductor layer each have a crystalline structure.

Since the proportion of indium in the second oxide semiconductor layeris higher than that in each of the first oxide semiconductor layer andthe third oxide semiconductor layer, the second oxide semiconductorlayer has high carrier mobility and serves as a carrier path. Hence, acarrier flows in a region apart from the oxide insulating layersprovided under and over the oxide semiconductor stack. Thus, an effectof impurities or the like entering from the oxide insulating layers canbe reduced.

Materials of the oxide semiconductor stack are selected as appropriateso that a well-shaped structure (also referred to as a well structure)is formed in which the bottom of the conduction band of the second oxidesemiconductor layer is at the deepest energy level. Specifically,materials may be selected as appropriate so that the bottom of theconduction band of the second oxide semiconductor layer is deeper fromthe vacuum level than the bottoms of the conduction bands of the firstand the third oxide semiconductor layers.

Further, when silicon or carbon which is one of Group 14 elements iscontained as an impurity in the oxide semiconductor layer, it can workas a donor and form an n-type region. Thus, the concentration of siliconcontained in the oxide semiconductor layer is made lower than or equalto 3×10¹⁸ atoms/cm³, preferably lower than or equal to 3×10¹⁷ atoms/cm³.Further, the concentration of carbon in the oxide semiconductor layer ismade lower than or equal to 3×10¹⁸ atoms/cm³, preferably lower than orequal to 3×10¹⁷ atoms/cm³. It is particularly preferable to employ astructure where the first and the third oxide semiconductor layerssandwich or surround the second oxide semiconductor layer serving as acarrier path so that a large number of Group 14 elements do not enterthe second oxide semiconductor layer. That is to say, the first and thethird oxide semiconductor layer can also be called barrier layers whichprevent Group 14 elements such as silicon from entering the second oxidesemiconductor layer.

When hydrogen or moisture is contained as an impurity in the oxidesemiconductor stack, it can work as a donor and form an n-type region.Therefore, in terms of achieving the well-shaped structure, it isadvantageous to provide a protective film (e.g., a silicon nitride film)that prevents entry of hydrogen or moisture from outside, under or overthe oxide semiconductor stack.

With the oxide semiconductor layers having the above stacked-layerstructure, a region where a channel is formed can have an absorptioncoefficient due to the localized states measured by a constantphotocurrent method (CPM) which is lower than or equal to 3×10⁻³/cm(lower than or equal to 3×10¹³/cm³ when converted into density ofstates).

A semiconductor device of one embodiment of the present inventionincludes a first oxide insulating layer formed over a semiconductorsubstrate, an oxide semiconductor stack in which a first oxidesemiconductor layer, a second oxide semiconductor layer, and a thirdoxide semiconductor layer are stacked over the first oxide insulatinglayer, a second oxide insulating layer over the oxide semiconductorstack, and a first gate electrode layer overlapping with the oxidesemiconductor stack with the second oxide insulating layer providedtherebetween. In the semiconductor device, the first to the third oxidesemiconductor layers each contain at least indium, the proportion ofindium in the second oxide semiconductor layer is higher than theproportion of indium in each of the first oxide semiconductor layer andthe third oxide semiconductor layer, the second oxide semiconductorlayer and the third oxide semiconductor layer each have a crystallinestructure, and the first oxide semiconductor layer is amorphous.

In addition to the above-described structure, a first nitride insulatinglayer may be provided under the first oxide semiconductor layer, and asecond nitride insulating layer may be provided over the second oxideinsulating layer. The first and the second nitride insulating layersprevent entry of hydrogen, moisture, or the like into the oxidesemiconductor stack.

The first oxide insulating layer and the second oxide insulating layermay contain oxygen in excess of the stoichiometric composition. Whenoxygen is thus contained in excess of the stoichiometric composition,oxygen can be supplied to the oxide semiconductor stack, so that anoxygen vacancy can be filled with oxygen.

In the first oxide semiconductor layer and the third oxide semiconductorlayer, the concentration of at least indium may be higher than or equalto 1×10¹⁹ atoms/cm³. Further, in the oxide semiconductor stack, theabsorption coefficient due to localized states may be lower than orequal to 3×10⁻³/cm.

The semiconductor device may include a second gate electrode layeroverlapping with the oxide semiconductor stack with the first oxideinsulating layer provided therebetween.

The second oxide semiconductor layer and the third oxide semiconductorlayer may each include a crystal whose c-axis is aligned in a directionapproximately perpendicular to a surface.

The first oxide semiconductor layer, the second oxide semiconductorlayer, and the third oxide semiconductor layer may each contain indium,zinc, and gallium. In particular, when the first oxide semiconductorlayer, the second oxide semiconductor layer, and the third oxidesemiconductor layer are formed using the same elements, scatterings atthe interface between the first and the second oxide semiconductorlayers and the interface between the second and the third oxidesemiconductor layers can be reduced.

The concentration of silicon in each of the first oxide semiconductorlayer and the third oxide semiconductor layer may be lower than or equalto 3×10¹⁸ atoms/cm³. The concentration of carbon in each of the firstoxide semiconductor layer and the third oxide semiconductor layer may belower than or equal to 3×10¹⁸ atoms/cm³.

Another embodiment of the present invention is a method for fabricatinga semiconductor device, including the following steps: forming a firstoxide insulating layer over a semiconductor substrate; over the firstoxide insulating layer, forming a first oxide semiconductor layer beingamorphous and a second oxide semiconductor layer having a crystallinestructure; performing first heat treatment in an atmosphere of oxygenand nitrogen; forming a third oxide semiconductor layer having acrystalline structure over the second oxide semiconductor layer; forminga second oxide insulating layer over the third oxide semiconductorlayer; and performing second heat treatment in an atmosphere of oxygenand nitrogen.

Another embodiment of the present invention is a method for fabricatinga semiconductor device, including the following steps: forming a firstoxide insulating layer over a semiconductor substrate; over the firstoxide insulating layer, stacking a first oxide semiconductor layer beingamorphous and a second oxide semiconductor layer having a crystallinestructure; and forming a third oxide semiconductor layer over the secondoxide semiconductor layer. Crystal growth of the third oxidesemiconductor layer using a crystal in the second oxide semiconductorlayer as a seed occurs.

Note that a semiconductor substrate provided with a transistor may beused as the semiconductor substrate.

According to one embodiment of the present invention, it is possible toprovide a highly reliable semiconductor device that includes an oxidesemiconductor and exhibits stable electrical characteristics. It ispossible to fabricate a highly reliable semiconductor device.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view illustrating a semiconductor deviceaccording to one embodiment of the present invention.

FIGS. 2A and 2B are views illustrating a method for fabricating asemiconductor device, according to one embodiment of the presentinvention.

FIGS. 3A and 3B are views illustrating a method for fabricating asemiconductor device, according to one embodiment of the presentinvention.

FIGS. 4A and 4B are cross-sectional views each illustrating asemiconductor device according to one embodiment of the presentinvention.

FIGS. 5A to 5C are cross-sectional views each illustrating asemiconductor device according to one embodiment of the presentinvention.

FIGS. 6A and 6B are circuit diagrams each illustrating a semiconductordevice according to one embodiment of the present invention.

FIGS. 7A to 7C are circuit diagrams and a conceptual diagram of asemiconductor device according to one embodiment of the presentinvention.

FIG. 8 is a block diagram of a semiconductor device according to oneembodiment of the present invention.

FIG. 9 is a block diagram of a semiconductor device according to oneembodiment of the present invention.

FIG. 10 is a block diagram of a semiconductor device according to oneembodiment of the present invention.

FIGS. 11A and 11B illustrate an electronic device in which asemiconductor device according to one embodiment of the presentinvention can be used.

FIG. 12A is a view illustrating an oxide semiconductor stack included ina semiconductor device according to one embodiment of the presentinvention, FIG. 12B is a band diagram of the oxide semiconductor stack,and FIG. 12C is a band diagram of an oxide semiconductor stack includedin a semiconductor device according to another embodiment of the presentinvention.

FIG. 13 is a top view illustrating an example of an apparatus forfabricating a semiconductor device.

FIG. 14A shows energy from a vacuum level to a bottom of a conductionband of an oxide semiconductor stack, and FIG. 14B is a band diagramthereof.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that modes and details thereofcan be modified in various ways. Therefore, the present invention is notconstrued as being limited to description of the embodiments.

Further, in embodiments hereinafter described, the same parts aredenoted with the same reference numerals throughout the drawings. Notethat the thickness, width, relative positional relation, and the like ofcomponents, i.e., a layer, a region, and the like, which are illustratedin the drawings are exaggerated for clarification of descriptions of theembodiments in some cases.

Note that the term such as “over” in this specification and the likedoes not necessarily mean that a component is placed “directly on”another component. For example, the expression “a gate electrode layerover an insulating layer” does not exclude the case where there is anadditional component between the insulating layer and the gate electrodelayer. The same applies to the term “below”.

In this specification and the like, the term “electrode layer” or“wiring layer” does not limit the function of components. For example,an “electrode layer” can be used as part of a “wiring layer”, and the“wiring layer” can be used as part of the “electrode layer”. Inaddition, the term “electrode layer” or “wiring layer” can also mean acombination of a plurality of “electrode layers” and “wiring layers”,for example.

Functions of a “source” and a “drain” are sometimes replaced with eachother when a transistor of opposite polarity is used or when thedirection of current flowing is changed in circuit operation, forexample. Therefore, the terms “source” and “drain” can be replaced witheach other in this specification.

Note that in this specification and the like, the term “electricallyconnected” includes the case where components are connected through anobject having any electric function. There is no particular limitationon an object having any electric function as long as electric signalscan be transmitted and received between components that are connectedthrough the object.

Examples of an “object having any electric function” are an electrodeand a wiring.

Embodiment 1

In this embodiment, a semiconductor device according to one embodimentof the present invention is described in detail with reference todrawings. FIG. 1 illustrates a semiconductor device of one embodiment ofthe present invention.

The semiconductor device includes a transistor 160 including a firstsemiconductor material, and a transistor 162 including a secondsemiconductor material formed over the transistor 160.

Here, the semiconductor materials included in the transistor 160 and thetransistor 162 preferably have different band gaps. For example, asilicon-based semiconductor (single crystal silicon, polycrystallinesilicon, or the like) can be used for the first semiconductor material,and an oxide semiconductor can be used for the second semiconductormaterial. A transistor including a silicon-based semiconductor caneasily operate at high speed. A transistor including an oxidesemiconductor, on the other hand, has low off-state current owing to itscharacteristics.

The transistor 160 illustrated in FIG. 1 includes a gate insulatinglayer 108 over the substrate 100 and a gate electrode layer 110 over thegate insulating layer 108. Note that the substrate 100 is provided witha channel formation region, a source region, and a drain region. Anelement isolation insulating layer 102 is provided to surround thetransistor 160. A wiring layer 112 that is electrically connected to thetransistor 160 is provided over the element isolation insulating layer102.

Note that a sidewall insulating layer may be provided in contact withthe side surface of the gate electrode layer 110 of the transistor 160.When the sidewall insulating layer is not provided as illustrated inFIG. 1, high integration can be achieved.

An insulating layer 104 is formed over the transistor 160, the elementisolation insulating layer 102, and the wiring layer 112. An openingreaching the wiring layer 112 is formed in the insulating layer 104. Awiring layer 114 is formed in the opening. The wiring layer 114 can beformed in such a way that, after the opening is formed in the insulatinglayer 104, a conductive film is formed in the opening, and theinsulating layer 104 and the conductive film are planarized by chemicalmechanical polishing (CMP) or the like.

A wiring layer 115 is formed over the insulating layer 104. The wiringlayer 115 has a stacked-layer structure of a wiring layer 115 a, awiring layer 115 b, and a wiring layer 115 c. However, the wiring layer115 is not limited to this structure and may be provided in accordancewith needed characteristics. An insulating layer 120 is provided overthe wiring layer 115. The top surface of the insulating layer 120 isflat; thus, unevenness due to the transistor 160 and the wiring layer115 is reduced.

An insulating layer 135 is provided over the insulating layer 120. Theinsulating layer 135 is formed using a film having a blocking propertyin order to inhibit the degradation in the characteristics of thetransistor 160 which is caused by the release of hydrogen or the likefrom the transistor 162 and to prevent impurities due to the transistor160 from entering the transistor 162. Here, examples of the impuritiesthat enter the transistor 162 include hydrogen, moisture, and nitrogen.Thus, a film that does not transmit these impurities is preferably usedfor the insulating layer 135.

In an opening formed in the insulating layer 135 and the insulatinglayer 120, a wiring layer 116 is formed in contact with the wiring layer115. A wiring layer 117 being in contact with the wiring layer 116 isformed over the insulating layer 135. An insulating layer 140 is formedover the wiring layer 117.

The insulating layer 140 may be a film which contains oxygen in excessof the stoichiometric composition. With the use of the insulating layer140 containing oxygen in excess of in the stoichiometric composition,oxygen can be supplied to an oxide semiconductor stack 144 that is incontact with the insulating layer 140. Thus, oxygen vacancies of theoxide semiconductor stack 144 can be reduced.

The top surface of the insulating layer 140 is subjected toplanarization treatment by chemical mechanical polishing or the like, sothat unevenness due to the transistor 160, the wiring layer 115, thewiring layer 117, and the like is reduced. By the improvement inplanarity of the top surface of the insulating layer 140, the oxidesemiconductor stack 144 can be made uniform in thickness distributionand the characteristics of the transistor 162 can be improved.

The transistor 162 is formed over the insulating layer 140. Thetransistor 162 includes the oxide semiconductor stack 144, a sourceelectrode layer 142 a and a drain electrode layer 142 b that are incontact with the oxide semiconductor stack 144, a gate insulating layer147 over the oxide semiconductor stack 144, the source electrode layer142 a, and the drain electrode layer 142 b, a gate electrode layer 148over the gate insulating layer 147, an insulating layer 150 over thegate electrode layer 148, and an insulating layer 155.

In the oxide semiconductor stack 144, a first oxide semiconductor layer144 a, a second oxide semiconductor layer 144 b, and a third oxidesemiconductor layer 144 c are stacked. The second oxide semiconductorlayer 144 b is formed using an oxide semiconductor having higher carrierdensity than the first oxide semiconductor layer 144 a and the thirdoxide semiconductor layer 144 c. Thus, a channel is formed in the secondoxide semiconductor layer 144 b having high carrier density, and theregion where the channel is formed can be apart from the interfacebetween the oxide semiconductor stack 144 and the insulating layer.

Further, the first oxide semiconductor layer 144 a is amorphous, and thesecond oxide semiconductor layer 144 b and the third oxide semiconductorlayer 144 c each have a crystalline structure. A semiconductor filmhaving a crystalline structure is used for the second oxidesemiconductor layer 144 b, whereby the effect of oxygen vacancies in thechannel formation region can be reduced.

Crystal growth of the third oxide semiconductor layer 144 c occurs usinga crystal in the second oxide semiconductor layer 144 b as a seed. Thus,in some cases, a region of the third oxide semiconductor layer 144 cwhich overlaps with the second oxide semiconductor layer 144 b has acrystalline structure and a region of the third oxide semiconductorlayer 144 c which does not overlap with the second oxide semiconductorlayer 144 b (a region being in contact with the insulating layer 140 andthe side surfaces of the first oxide semiconductor layer 144 a) has anamorphous structure. Hence, in drawings, the hatch patterns of theregion of the third oxide semiconductor layer 144 c which overlaps withthe second oxide semiconductor layer 144 b and the other region of thethird oxide semiconductor layer 144 c are different from each other.

Note that the second oxide semiconductor layer 144 b and the third oxidesemiconductor layer 144 c each have a crystalline structure and theinterface between the second oxide semiconductor layer 144 b and thethird oxide semiconductor layer 144 c is not clearly observed in somecases. Hence, the interface between the second oxide semiconductor layer144 b and the third oxide semiconductor layer 144 c is indicated by adotted line in drawings.

Next, a method for fabricating the semiconductor device of oneembodiment of the present invention is described. First, an insulatingfilm which is to be the gate insulating layer 108 is formed over thesubstrate 100.

A single crystal semiconductor substrate or a polycrystallinesemiconductor substrate of silicon, silicon carbide, or the like or acompound semiconductor substrate of silicon germanium or the like may beused as the substrate 100. Alternatively, an SOI substrate, asemiconductor substrate over which a semiconductor element is provided,or the like can be used.

Further alternatively, a glass substrate of barium borosilicate glass,aluminoborosilicate glass, or the like, a ceramic substrate, a quartzsubstrate, a sapphire substrate, or the like over which a semiconductorlayer is formed by a sputtering method, a vapor phase growth method suchas a plasma CVD method, or the like may be used. As the semiconductorlayer, any of the following can be used: amorphous silicon;polycrystalline silicon obtained by crystallization of amorphous siliconby laser annealing or the like; single crystal silicon obtained in sucha manner that a surface portion of a single crystal silicon wafer isseparated after implantation of hydrogen ions or the like into thesilicon wafer; and the like. Any of these semiconductor layers may beprocessed into an island-like shape by a photolithography step.

A protective layer which is to be a mask for forming the elementisolation insulating layer is formed, and etching is performed with theuse of the protective layer as a mask, whereby a part of the substrate100 which is not covered with the protective layer is removed. Thus, aplurality of isolated semiconductor regions is formed in an upperportion of the substrate 100. After an insulating layer is formed tocover the isolated semiconductor regions, the insulating layer whichoverlaps with the semiconductor regions is selectively removed. In thismanner, the element isolation insulating layer 102 is formed.

Next, the gate insulating layer 108 and the gate electrode layer 110 arestacked. The gate electrode layer 108 can be formed by a sputteringmethod, a molecular beam epitaxy (MBE) method, a chemical vapordeposition (CVD) method, a pulsed laser deposition (PLD) method, anatomic layer deposition (ALD) method, or the like, as appropriate. Whenthe gate electrode layer 108 is formed by a sputtering method, animpurity element such as hydrogen can be reduced.

The gate insulating layer 108 can be formed using an inorganicinsulating film. It is preferable to use, for example, a silicon oxidefilm, a silicon oxynitride film, an aluminum oxide film, an aluminumoxynitride film, a hafnium oxide film, a gallium oxide film, a siliconnitride film, an aluminum nitride film, a silicon nitride oxide film, oran aluminum nitride oxide film. Further, the gate insulating layer 108can be formed with a single-layer structure or a stacked-layer structureincluding two or more layers with the use of these compounds.

The gate electrode layer 110 (and the wiring layer 112 formed using thesame conductive film as the gate electrode layer 110, and the like) maybe formed using a metal material such as molybdenum, titanium, tantalum,tungsten, aluminum, copper, chromium, neodymium, or scandium, or analloy material containing any of these materials as its main componentby a plasma CVD method, a sputtering method, or the like. The gateelectrode layer 110 may also be formed using a semiconductor filmtypified by a polycrystalline silicon film doped with an impurityelement such as phosphorus, or a silicide film of nickel silicide or thelike. Further, the gate electrode layer 110 may also be formed using aconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded. It is also possible that the gate electrode layer 110 has astacked-layer structure of the above conductive material and the abovemetal material.

Here, with the use of the gate electrode layer 110 as a mask, animpurity element imparting n-type conductivity or an impurity elementimparting p-type conductivity is introduced to the substrate 100. Thus,a source region and a drain region are formed. As the method forintroducing the impurity element, an ion implantation method, an iondoping method, a plasma immersion ion implantation method, or the likecan be used.

Phosphorus, boron, nitrogen, arsenic, argon, aluminum, a molecular ioncontaining any of these elements, or the like can be used as theimpurity element to be introduced. The dosage of such an element ispreferably 1×10¹³ ions/cm² to 5×10¹⁶ ions/cm². When phosphorus isintroduced as the impurity element, the acceleration voltage ispreferably 0.5 kV to 80 kV.

Note that the treatment for introducing the impurity element may beperformed plural times. In the case where the treatment for introducingthe impurity element is performed plural times, the kind of impurityelement may be the same in the plural treatments or different in everytreatment.

By the above process, the transistor 160 can be fabricated.

Next, the insulating layer 104 is formed to cover the gate electrodelayer 110, the gate insulating layer 108, the element isolationinsulating layer 102, and the wiring layer 112. The insulating layer 104can be formed using a material and a method which are similar to thoseof the gate insulating layer 108.

Further, an opening is formed in the insulating layer 104, and thewiring layer 114 is formed in the opening (see FIG. 2A). The wiringlayer 114 can be formed using a material and a method which are similarto those of the wiring layer 112.

Next, the wiring layer 115 is formed over the insulating layer 104. Thewiring layer 115 can be formed using a material and a method which aresimilar to those of the wiring layer 112.

Here, in order to reduce the resistance of the wiring layer 115 andallow the wiring layer 115 to have heat resistance, the wiring layer 115has a three-layer structure. In the three-layer structure, an aluminumfilm having low resistivity is used as the wiring layer 115 b, andtitanium films each having a high melting point are formed as the wiringlayer 115 a and the wiring layer 115 c over and under the aluminum film.

Note that after conductive films to be the wiring layer 115 are formed,the conductive films are etched. In the step of etching the conductivefilms, the insulating layer 104 is also etched concurrently and isreduced in thickness in some cases. Hence, a region of the insulatinglayer 104 which overlaps with the wiring layer 115 has a largerthickness than the other region in some cases. Thus, a surface of theinsulating layer 104 sometimes has unevenness.

Next, the insulating layer 120 is formed over the insulating layer 104and the wiring layer 115. The insulating layer 120 can be formed usingan inorganic material which is similar to that of the gate insulatinglayer 108 or an organic material such as a polyimide resin, an acrylicresin, or a benzocyclobutene-based resin so that unevenness due to thetransistor 160, the insulating layer 104, and the wiring layer 115 isreduced. Other than such organic materials, it is also possible to use alow dielectric constant material (low-k material) or the like.Alternatively, the insulating layer 120 may be formed by stacking aplurality of insulating films formed using any of these materials.

Next, the insulating layer 135 is formed over the insulating layer 120.

The insulating layer 135 is preferably formed using a film having ablocking property so that impurities due to the transistor 160 do notenter the transistor 162. For example, the insulating layer 135 may beformed using a film containing silicon nitride, aluminum oxide, aluminumoxynitride, gallium oxide, gallium oxynitride, yttrium oxide, yttriumoxynitride, hafnium oxide, hafnium oxynitride, or the like.

Further, in the case where the insulating layer 135 includes a portionhaving low density or a portion where a film is not formed (hereinafterthese portions are also collectively referred to as “cavity”),impurities enter the transistor through the cavity in some cases.

The planarity of the insulating layer 120 may be improved in advance sothat the cavity is not formed in the insulating layer 135. For example,planarization treatment such as chemical mechanical polishing treatmentor plasma treatment may be performed on a surface of the insulatinglayer 120 before the insulating layer 135 is formed.

After the insulating layer 135 is formed, an opening reaching the wiringlayer 115 is formed in the insulating layer 135 and the insulating layer120. The wiring layer 116 is formed in the opening. Over the insulatinglayer 135, the wiring layer 117 is formed in contact with the wiringlayer 116. The wiring layers 116 and 117 can be formed using a materialand a method which are similar to those of the gate electrode layer 110.

Next, the insulating layer 140 is formed over the wiring layer 117 (seeFIG. 2B). The insulating layer 140 may be formed using an oxideinsulating layer containing nitrogen or an oxide insulating layer suchas a silicon oxide film, a silicon oxynitride film, an aluminum oxidefilm, an aluminum oxynitride film, a hafnium oxide film, or a galliumoxide film. Further, the insulating layer 140 can be formed with asingle-layer structure or a stacked-layer structure including two ormore layers with the use of these compounds. A film containing oxygen inexcess of the stoichiometric composition may be used for the insulatinglayer 140 so as to supply oxygen to the oxide semiconductor stack 144 tobe formed later.

Further, the insulating layer 140 may be planarized by chemicalmechanical polishing treatment or the like so that the oxidesemiconductor stack 144 to be formed later is made uniform in filmthickness distribution and is improved in crystallinity.

Next, the oxide semiconductor stack 144 is formed over the insulatinglayer 140.

Each of the first to the third oxide semiconductor layers 144 a to 144 cis formed using a sputtering target which contains at least indium (In)and which allows film formation to be performed by an AC sputteringmethod or a DC sputtering method. By containing indium, the sputteringtarget can have increased conductivity. With the use of such asputtering target, film formation by an AC sputtering method or a DCsputtering method is performed more easily. The target may be such thata film formed with the use of the target contains indium at aconcentration higher than or equal to 1×10¹⁹ atoms/cm³ at least afterthe film is formed. The first oxide semiconductor layer 144 a and thethird oxide semiconductor layer 144 c are each formed using a materialthat can be represented by InM1_(X)Zn_(Y)O_(Z) (X≧1, Y>1, Z>0, and M1 isGa, Hf, or the like). Note that in the case where the first oxidesemiconductor layer 144 a and the third oxide semiconductor layer 144 care each formed using a material containing Ga, a material containing ahigh proportion of Ga, or more specifically, a material that can berepresented by InM1_(X)Zn_(Y)O_(Z) where X is larger than 10 is notsuitable because, in that case, dust might be generated at the time offilm formation and it is difficult to perform film formation by an ACsputtering method or a DC sputtering method.

The second oxide semiconductor layer 144 b is formed using a materialthat can be represented by InM2_(X)Zn_(Y)O_(Z) (X≧1, Y≧X, Z>0, M2=Ga,Sn, or the like).

In particular, in the case where the second oxide semiconductor layer144 b is In-M2-Zn oxide (M2 is Ga, Sn, or the like) and a target havingthe atomic ratio of metal elements of In:M2:Zn=x₁:y₁:z₁ is used forforming the second oxide semiconductor layer 144 b, x₁/y₁ is preferablygreater than or equal to ⅓ and less than or equal to 6, furtherpreferably greater than or equal to 1 and less than or equal to 6, andz₁/y₁ is preferably greater than or equal to ⅓ and less than or equal to6, further preferably greater than or equal to 1 and less than or equalto 6. Note that when z₁/y₁ is greater than or equal to 1 and less thanor equal to 6, a CAAC-OS film to be described later as the second oxidesemiconductor layer 144 b is easily formed. Typical examples of theatomic ratio of the metal elements of the target are In:M2:Zn=1:1:1 andIn:M2:Zn=3:1:2.

Further, particularly in the case where each of the first oxidesemiconductor layer 144 a and the third oxide semiconductor layer 144 cis In-M1-Zn oxide (M1 is Ga, Hf, or the like) and a target having theatomic ratio of the metal elements of In:M1:Zn=x₂:y₂:z₂ is used forforming each of the first oxide semiconductor layer 144 a and the thirdoxide semiconductor layer 144 c, it is preferable that x₂/y₂<x₁/y₁ andthat z₂/y₂ is greater than or equal to ⅓ and less than or equal to 6,further preferably greater than or equal to 1 and less than or equal to6. Note that when z₂/y₂ is greater than or equal to 1 and less than orequal to 6, CAAC-OS films to be described later are easily formed as thefirst oxide semiconductor layer 144 a and the third oxide semiconductorlayer 144 c. Typical examples of the atomic ratio of the metal elementsof the target are In:M1:Zn=1:3:2, In:M1:Zn=1:3:4, In:M1:Zn=1:3:6,In:M1:Zn=1:3:8, and the like.

As the first to the third oxide semiconductor layers 144 a to 144 c,oxide having the atomic ratio of In:Ga:Zn=1:1:1 (=⅓:⅓:⅓), In:Ga:Zn=2:2:1(=⅖:⅖:⅕), In:Ga:Zn=3:1:2 (=½:⅙:⅓), In:Ga:Zn=1:3:2 (=⅙:½:⅓),In:Ga:Zn=1:4:3 (=⅛:½:⅜), In:Ga:Zn=1:5:4 (= 1/10:½:⅖), or In:Ga:Zn=1:6:6(= 1/13: 6/13: 6/13), or the like may be used. Note that the first andthe third oxide semiconductor layers 144 a and 144 c may be formed usinghafnium instead of gallium. Further, the second oxide semiconductorlayer 144 b may be formed using tin instead of gallium.

Materials of the first to the third oxide semiconductor layers areselected as appropriate so that a well-shaped structure is formed inwhich the bottom of the conduction band of the second oxidesemiconductor layer 144 b is deeper from the vacuum level than thebottoms of the conduction bands of the first and the third oxidesemiconductor layers 144 a and 144 c. The depth of a bottom of aconduction band from a vacuum level (such depth can also be expressed aselectron affinity) can be obtained by subtracting a difference in energybetween the bottom of the conduction band and a top of a valence band(what is called a band gap) from a difference in energy between thevacuum level and the top of the valence band (what is called anionization potential).

Note that the ionization potential of an oxide semiconductor which isused for obtaining electron affinity can be measured by ultravioletphotoelectron spectroscopy (UPS) or the like. As a typical measurementdevice of UPS, VersaProbe (manufactured by ULVAC-PHI, Inc) is used. Theelectron affinity refers to a difference in energy between the vacuumlevel (E_(∞)) and an end of the conduction band (E_(c)). Further, energyband gap (E_(g)) can be measured by a full automatic spectroscopicellipsometer UT-300. The energy of the bottom of the conduction band isobtained by subtracting the energy band gap from the value of theionization potential, and thus, the band structure of a single layer orstacked layers can be formed. By using this method, it can be confirmedwhether a buried channel is formed with the use of the stacked-layerstructure disclosed in this specification. FIGS. 14A and 14B eachillustrate an example thereof.

FIG. 14A shows data of energy from the vacuum level to the bottom of theconduction band. FIG. 14B shows the band structure formed on the basisof the data. To obtain the data, a sample with a stacked-layer structureis formed in such a manner that, after a 10 nm thick film is formedusing an In—Ga—Zn oxide sputtering target having a composition ofIn:Ga:Zn=1:1:1 [atomic ratio] under an atmosphere where the proportionof oxygen is 100%, a 10 nm thick film is formed using an In—Ga—Zn oxidesputtering target having a composition of In:Ga:Zn=3:1:2 [atomic ratio]under an atmosphere where the proportion of argon is 100%, and then, a10 nm thick film is formed using an In—Ga—Zn oxide sputtering targethaving a composition of In:Ga:Zn=1:1:1 [atomic ratio] under anatmosphere where the proportion of oxygen is 100%. With the use of thesample, an ionization potential is measured, and an energy band gap ismeasured by a full automatic spectroscopic ellipsometer UT-300. Theenergy from the vacuum level to the bottom of the conduction band isobtained by subtracting the energy band gap from the ionizationpotential. It is found from FIG. 14B that a well-shaped structure isformed in which the bottom of the conduction band of the second oxidesemiconductor layer is deeper from the vacuum level than the bottoms ofthe conduction bands of the first and the third oxide semiconductorlayers.

Further, in the case where the first to the third oxide semiconductorlayers 144 a to 144 c are formed using an In—Ga—Zn oxide, theconstituent elements of the first to the third oxide semiconductorlayers 144 a to 144 c are the same. Hence, the number of trap levels atthe interface between the first oxide semiconductor layer 144 a and thesecond oxide semiconductor layer 144 b and the interface between thesecond oxide semiconductor layer 144 b and the third oxide semiconductorlayer 144 c is small. Thus, it is possible to reduce a change of atransistor over time and a change in threshold voltage due to stresstest.

In Ga, the formation energy of oxygen vacancies is larger and thusoxygen vacancies are less likely to occur, than in In; therefore, theoxide having a composition in which the proportion of In is equal to orlower than that of Ga has more stable characteristics than the oxidehaving a composition in which the proportion of In is higher than thatof Ga. Hence, the interface between the first oxide semiconductor layer144 a and a silicon insulating layer and the interface between the thirdoxide semiconductor layer 144 c and a silicon insulating layer can bestable. Thus, a highly reliable semiconductor device can be obtained.

In an oxide semiconductor, the s orbital of heavy metal mainlycontributes to carrier transfer, and when the proportion of In in theoxide semiconductor is increased, overlap of the s orbital is likely tobe increased. Therefore, oxide having a composition in which theproportion of In is higher than that of Ga has higher mobility thanoxide having a composition in which the proportion of In is equal to orlower than that of Ga. Hence, when a carrier is formed in the secondoxide semiconductor layer 144 b containing a high proportion of indium,high mobility can be achieved.

A material of the second oxide semiconductor layer 144 b is selected asappropriate so that the bottom of the conduction band of the secondoxide semiconductor layer 144 b forms a well-shaped structure. Anexample of the well-shaped structure is illustrated in FIG. 12B. FIG.12B is an energy band diagram along Y1-Y2 in a transistor illustrated inFIG. 12A. Note that the transistor illustrated in FIG. 12A has astructure similar to that of a transistor 163 illustrated in FIG. 4A;therefore, a detailed description thereof is omitted.

Here, when silicon or carbon which is one of Group 14 elements iscontained as an impurity in the oxide semiconductor layer, it can workas a donor and form an n-type region. Thus, the concentration of siliconin each oxide semiconductor layer is less than or equal to 3×10¹⁸atoms/cm³, preferably less than or equal to 3×10¹⁷ atoms/cm³. Further,the concentration of carbon is less than or equal to 3×10¹⁸ atoms/cm³,preferably less than or equal to 3×10¹⁷ atoms/cm³. In particular, it ispreferable to use a structure in which the first oxide semiconductorlayer 144 a and the third oxide semiconductor layer 144 c sandwich orsurround the second oxide semiconductor layer 144 b to be a carrier pathso that Group 14 elements are prevented from being contained at a highproportion in the second oxide semiconductor layer 144 b. That is tosay, the first and the third oxide semiconductor layer 144 a and 144 ccan also be called barrier layers which prevent Group 14 elements suchas silicon from entering the second oxide semiconductor layer 144 b.

Hydrogen contained in the oxide semiconductor stack 144 reacts withoxygen bonded to metal to produce water, and a defect is formed in alattice from which oxygen is released (or a portion from which oxygen isremoved). In addition, a bond of a part of hydrogen and oxygen causesgeneration of electrons serving as carriers. Thus, the impuritiescontaining hydrogen are reduced as much as possible in the step offorming the oxide semiconductor stack 144, whereby the hydrogenconcentration in the oxide semiconductor stack 144 can be reduced. Whenthe oxide semiconductor stack 144 which is highly purified by removinghydrogen as much as possible is used as a channel formation region, ashift of the threshold voltage in the negative direction can be reduced,and the leakage current between a source and a drain of the transistor(typically, the off-state current or the like) can be decreased toseveral yoctoamperes per micrometer to several zeptoamperes permicrometer. As a result, electric characteristics of the transistor canbe improved.

When the oxide semiconductor films to be semiconductor layers of thetransistor have the above-described stacked-layer structure, in a regionwhere a channel is formed, an absorption coefficient due to thelocalized states measured by a constant photocurrent method (CPM) can belower than or equal to 3×10⁻³/cm (lower than or equal to 3×10¹³/cm³ whenconverted into density of states).

The above-described stacked-layer structure is an example in which onewell-shaped structure is formed using the first to the third oxidesemiconductor layers; however, the present invention is not limited tothe above-described stacked layer structure. A plurality of well-shapedstructures may be formed using the second oxide semiconductor layer witha multilayer structure. FIG. 12C illustrates an example thereof.

As a sputtering gas, a rare gas (typically argon) atmosphere, an oxygenatmosphere, or a mixed gas of a rare gas and oxygen is used asappropriate. In the case of using the mixed gas of a rare gas andoxygen, the proportion of oxygen is preferably higher than that of arare gas.

The target for forming the oxide semiconductor layer may be selected asappropriate in accordance with the composition of the oxidesemiconductor layer to be formed.

As an example of the target, an In—Ga—Zn oxide target is describedbelow.

The In—Ga—Zn oxide target, which is polycrystalline, is made by mixingInO_(X) powder, GaO_(Y) powder, and ZnO_(Z) powder in a predeterminedmolar ratio, applying pressure, and performing heat treatment at atemperature higher than or equal to 1000° C. and lower than or equal to1500° C. Note that X, Y, and Z are each a given positive number. Here,the predetermined molar ratio of InO_(X) powder to GaO_(Y) powder andZnO_(Z) powder is, for example, 2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3, or3:1:2. The kinds of powder and the molar ratio for mixing powder may bedetermined as appropriate depending on the desired target.

The second oxide semiconductor layer 144 b and the third oxidesemiconductor layer 144 c may have different crystallinities. That is,any of a single crystal oxide semiconductor film, a polycrystallineoxide semiconductor film, a microcrystalline oxide semiconductor film,and a CAAC-OS film may be combined as appropriate.

Here, the details of the CAAC-OS film are described. The CAAC-OS film isone of oxide semiconductor films having a plurality of c-axis alignedcrystal parts. In an image obtained with a transmission electronmicroscope (TEM), a grain boundary in the CAAC-OS film cannot be found.Thus, in the CAAC-OS film, a reduction in electron mobility due to thegrain boundary is less likely to occur.

In this specification, a term “parallel” indicates that the angle formedbetween two straight lines is greater than or equal to −10° and lessthan or equal to 10°, and accordingly also includes the case where theangle is greater than or equal to −5° and less than or equal to 5°. Inaddition, a term “perpendicular” indicates that the angle formed betweentwo straight lines is greater than or equal to 80° and less than orequal to 100°, and accordingly includes the case where the angle isgreater than or equal to 85° and less than or equal to 95°.

In this specification, the trigonal and rhombohedral crystal systems areincluded in the hexagonal crystal system.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface over which theCAAC-OS film is formed (hereinafter, a surface over which the CAAC-OSfilm is formed is referred to as a formation surface) or a top surfaceof the CAAC-OS film, and is arranged in parallel to the formationsurface or the top surface of the CAAC-OS film.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (plan TEM image), metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

Most of the crystal parts included in the CAAC-OS film each fit inside acube whose one side is less than 100 nm. Thus, there is a case where acrystal part included in the CAAC-OS film fits a cube whose one side isless than 10 nm, less than 5 nm, or less than 3 nm. Note that when aplurality of crystal parts included in the CAAC-OS film are connected toeach other, one large crystal region is formed in some cases. Forexample, a crystal region with an area of 2500 nm² or more, 5 μm² ormore, or 1000 μm² or more is observed in some cases in the plan TEMimage.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction substantiallyperpendicular to the c-axis, a peak appears frequently when 2θ is around56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal.Here, analysis (φ scan) is performed under conditions where the sampleis rotated around a normal vector of a sample surface as an axis (φaxis) with 2θ fixed at around 56°. In the case where the sample is asingle-crystal oxide semiconductor film of InGaZnO₄, six peaks appear.The six peaks are derived from crystal planes equivalent to the (110)plane. On the other hand, in the case of a CAAC-OS film, a peak is notclearly observed even when φ scan is performed with 2θ fixed at around56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned in adirection parallel to a normal vector of a formation surface or a normalvector of a top surface. Thus, for example, in the case where a shape ofthe CAAC-OS film is changed by etching or the like, the c-axis might notbe necessarily parallel to a normal vector of a formation surface or anormal vector of a top surface of the CAAC-OS film.

Further, distribution of c-axis aligned crystal parts in the CAAC-OSfilm is not necessarily uniform. For example, in the case where crystalgrowth leading to the crystal parts of the CAAC-OS film occurs from thevicinity of the top surface of the film, the proportion of the c-axisaligned crystal parts in the vicinity of the top surface is higher thanthat in the vicinity of the formation surface in some cases. Further,when an impurity is added to the CAAC-OS film, a region to which theimpurity is added is altered, and the proportion of the c-axis alignedcrystal parts in the CAAC-OS film varies depending on regions, in somecases.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ do not appear at around36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a decrease in crystallinity. Further, a heavy metalsuch as iron or nickel, argon, carbon dioxide, or the like has a largeatomic radius (molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier generationsources, and thus can have a low carrier density. Thus, a transistorincluding the oxide semiconductor film rarely has negative thresholdvoltage (is rarely normally on). The highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has fewcarrier traps. Accordingly, the transistor including the oxidesemiconductor film has little variation in electrical characteristicsand high reliability. Electric charge trapped by the carrier traps inthe oxide semiconductor film takes a long time to be released, and mightbehave like fixed electric charge. Thus, the transistor which includesthe oxide semiconductor film having high impurity concentration and ahigh density of defect states has unstable electrical characteristics insome cases.

With the use of the CAAC-OS film in a transistor, variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

In an oxide semiconductor having a crystal part as the CAAC-OS film,defects in the bulk can be further reduced and when the surface flatnessof the oxide semiconductor is improved, mobility higher than that of anoxide semiconductor in an amorphous state can be obtained. In order toimprove the surface planarity, the oxide semiconductor is preferablyformed over a flat surface.

When a CAAC-OS film is formed, for example, the CAAC-OS film is formedby a sputtering method using a polycrystalline oxide semiconductorsputtering target. By collision of ions with the target, a crystalregion included in the target may be separated from the target along ana-b plane; in other words, a sputtered particle having a plane parallelto an a-b plane (flat-plate-like sputtered particle or pellet-likesputtered particle) may flake off from the target. In that case, theflat-plate-like sputtered particle reaches a substrate while maintainingtheir crystal state, whereby the CAAC-OS film can be formed.

For the deposition of the CAAC-OS film, the following conditions arepreferably used.

By reducing the amount of impurities entering the CAAC-OS film duringthe deposition, the crystal state can be prevented from being broken bythe impurities. For example, the concentration of impurities (e.g.,hydrogen, water, carbon dioxide, or nitrogen) which exist in thedeposition chamber may be reduced. Furthermore, the concentration ofimpurities in a deposition gas may be reduced. Specifically, adeposition gas whose dew point is −80° C. or lower, preferably −100° C.or lower is used.

By increasing the substrate heating temperature during the deposition,migration of a sputtered particle is likely to occur after the sputteredparticle reaches a substrate surface. Specifically, the substrateheating temperature during the deposition is higher than or equal to100° C. and lower than or equal to 740° C., preferably higher than orequal to 200° C. and lower than or equal to 500° C. By increasing thesubstrate heating temperature during the deposition, when theflat-plate-like sputtered particle reaches the substrate, migrationoccurs on the substrate surface, so that a flat plane of theflat-plate-like sputtered particle is attached to the substrate.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is 30 vol % or higher, preferably 100 vol %.

Next, a microcrystalline oxide semiconductor film will be described.

In an image obtained with the TEM, crystal parts cannot be found clearlyin the microcrystalline oxide semiconductor in some cases. In mostcases, a crystal part in the microcrystalline oxide semiconductor isgreater than or equal to 1 nm and less than or equal to 100 nm, orgreater than or equal to 1 nm and less than or equal to 10 nm. Amicrocrystal with a size greater than or equal to 1 nm and less than orequal to 10 nm, or a size greater than or equal to 1 nm and less than orequal to 3 nm is specifically referred to as nanocrystal (nc). An oxidesemiconductor film including nanocrystal is referred to as an nc-OS(nanocrystalline oxide semiconductor) film. In an image obtained withTEM, a crystal grain cannot be found clearly in the nc-OS film in somecases.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic order. Further, there is noregularity of crystal orientation between different crystal parts in thenc-OS film; thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor film depending on an analysis method.For example, when the nc-OS film is subjected to structural analysis byan out-of-plane method with an XRD apparatus using an X-ray having abeam diameter larger than that of a crystal part, a peak which shows acrystal plane does not appear. Further, a halo-like pattern is shown ina selected-area electron diffraction pattern of the nc-OS film obtainedby using an electron beam having a probe diameter (e.g., larger than orequal to 50 nm) larger than that of a crystal part. Meanwhile, spots areshown in a nanobeam electron diffraction pattern of the nc-OS filmobtained by using an electron beam having a probe diameter (e.g., largerthan or equal to 1 nm and smaller than or equal to 30 nm) close to, orsmaller than that of a crystal part. Further, in a nanobeam electrondiffraction pattern of the nc-OS film, regions with high luminance in acircular (ring) pattern are shown in some cases. Also in a nanobeamelectron diffraction pattern of the nc-OS film, a plurality of spots isshown in a ring-like region in some cases.

Since the nc-OS film is an oxide semiconductor film having moreregularity than the amorphous oxide semiconductor film, the nc-OS filmhas a lower density of defect states than the amorphous oxidesemiconductor film. However, there is no regularity of crystalorientation between different crystal parts in the nc-OS film; hence,the nc-OS film has a higher density of defect states than the CAAC-OSfilm.

Note that an oxide semiconductor film may be a stacked film includingtwo or more films of an amorphous oxide semiconductor film, amicrocrystalline oxide semiconductor film, and a CAAC-OS film, forexample.

Here, by the above-described method, the first oxide semiconductor layer144 a and the second oxide semiconductor layer 144 b are stacked, andthen, heat treatment is performed, and selective etching is performedwith the use of a mask.

In this embodiment, the first oxide semiconductor layer 144 a having anamorphous structure is formed under the condition where the substratetemperature is set to room temperature and a target having an atomicratio of In:Ga:Zn=1:3:2 is used. The thickness of the first oxidesemiconductor layer 144 a having an amorphous structure is set to begreater than or equal to 10 nm and less than or equal to 40 nm,preferably greater than or equal to 20 nm and less than or equal to 30nm. The first oxide semiconductor layer 144 a having an amorphousstructure has such a large film thickness; thus, the diffusion ofsilicon from a base film (an insulating film containing silicon) can beprevented. Further, the second oxide semiconductor layer 144 b having acrystalline structure is formed under the condition where the substratetemperature is set to 400° C. and a target having an atomic ratio ofIn:Ga:Zn=1:1:1 is used. The second oxide semiconductor layer 144 b is afilm containing a crystal whose c-axis is aligned in a directionapproximately perpendicular to a surface, preferably a CAAC-OS film. Thesecond oxide semiconductor layer 144 b has a thickness greater than orequal to 5 nm and less than or equal to 10 nm.

The second oxide semiconductor layer 144 b having a crystallinestructure is stacked over the first oxide semiconductor layer 144 ahaving an amorphous structure. Hence, the stack can be also referred toas a heterostructure.

The film formation temperature of the second oxide semiconductor layer144 b is higher than or equal to 400° C. and lower than or equal to 550°C., preferably higher than or equal to 450° C. and lower than or equalto 500° C. Note that the film formation is performed at temperaturesthat the wiring layers already formed can withstand.

The heat treatment after the film formation is performed under reducedpressure in an atmosphere of nitrogen, oxygen, or nitrogen and oxygen ata temperature higher than or equal to 150° C. and lower than the strainpoint of the substrate, preferably higher than or equal to 300° C. andlower than or equal to 500° C., further preferably higher than or equalto 350° C. and lower than or equal to 450° C. By the heat treatment,excess hydrogen (including water or a hydroxyl group) in the oxidesemiconductor layer is removed (dehydration or dehydrogenation).Further, a high-purity oxygen gas or ultra dry air (the moisture amountis less than or equal to 20 ppm (−55° C. by conversion into a dewpoint), preferably less than or equal to 1 ppm, further preferably lessthan or equal to 10 ppb, in the measurement with use of a dew pointmeter of a cavity ring down laser spectroscopy (CRDS) system) may beintroduced into the same furnace while the heating temperature after thetermination of the heat treatment is maintained or slow cooling isperformed to lower the temperature from the heating temperature. By theeffect of the oxygen gas, oxygen which is a main component of the oxidesemiconductor and which has been reduced at the same time as the stepfor removing impurities by dehydration or dehydrogenation is supplied.

The heat treatment is performed after the second oxide semiconductorlayer 144 b is formed, whereby the hydrogen concentration in the secondoxide semiconductor layer 144 b can be lower than 5×10¹⁸ atoms/cm³,preferably lower than or equal to 1×10¹⁸ atoms/cm³, further preferablylower than or equal to 5×10¹⁷ atoms/cm³, still further preferably lowerthan or equal to 1×10¹⁶ atoms/cm³.

The heat treatment is performed under an inert gas atmosphere containingnitrogen or a rare gas such as helium, neon, argon, xenon, or krypton.Further, the heat treatment may be performed under an inert gasatmosphere first, and then under an oxygen atmosphere. It is preferablethat the above inert gas atmosphere and the above oxygen atmosphere donot contain hydrogen, water, and the like. The treatment time is 3minutes to 24 hours. The number of times of the heat treatment performedon the oxide semiconductor layer is not limited, and the timing of theheat treatment is not limited.

Note that heating may be performed with the oxide insulating layerprovided over and/or under the oxide semiconductor stack 144, in whichcase oxygen is supplied from the oxide insulating layer to the oxidesemiconductor stack 144 to reduce oxygen defects in the oxidesemiconductor stack 144. The reduction of the oxygen defects in theoxide semiconductor stack 144 leads to favorable semiconductorcharacteristics.

Next, the third oxide semiconductor layer 144 c is formed to cover thetop surface and the side surfaces of the second oxide semiconductorlayer 144 b and the side surfaces of the first oxide semiconductor layer144 a (see FIG. 3A). The heat treatment for dehydration ordehydrogenation of the oxide semiconductor may be performed also afterthe third oxide semiconductor layer 144 c is formed.

The third oxide semiconductor layer 144 c is formed using a targethaving an atomic ratio of In:Ga:Zn=1:3:2 at a substrate temperature of400° C. The third oxide semiconductor layer 144 c is formed over thesecond oxide semiconductor layer 144 b having a crystalline structure,whereby crystal growth of the third oxide semiconductor layer 144 coccurs using a crystal in the second oxide semiconductor layer as aseed. Thus, the third oxide semiconductor layer 144 c easily becomes afilm having a crystalline structure. Accordingly, the boundary betweenthe second oxide semiconductor layer 144 b and the third oxidesemiconductor layer 144 c is difficult to determine in a cross-sectionalTEM image in some cases. In drawings, the interface between the secondoxide semiconductor layer 144 b and the third oxide semiconductor layer144 c is indicated by a dotted line.

A part of the third oxide semiconductor layer 144 c, i.e. a region ofthe third oxide semiconductor layer 144 c which is in contact with theinsulating layer 140 and does not overlap with the second oxidesemiconductor layer 144 b easily has an amorphous structure. Thethickness of the third oxide semiconductor layer 144 c is greater thanor equal to 10 nm and less than or equal to 40 nm, preferably greaterthan or equal to 20 nm and less than or equal to 30 nm. Further, in thethird oxide semiconductor layer 144 c, a region which overlaps with thesecond oxide semiconductor layer 144 b has a crystalline structure, andthe other region has an amorphous structure. In order to illustrate thisclearly in drawings, the hatch patterns of the region of the third oxidesemiconductor layer 144 c which overlaps with the second oxidesemiconductor layer 144 b and the other region of the third oxidesemiconductor layer 144 c are different from each other.

Note that the third oxide semiconductor layer 144 c has lowercrystallinity than the second oxide semiconductor layer 144 b. Hence, itcan be said that the boundary can be determined by the degree ofcrystallinity. Further, in the case where an oxide semiconductor layerhaving a crystalline structure is provided as the third oxidesemiconductor layer 144 c over the second oxide semiconductor layer 144b and the third oxide semiconductor layer 144 c has a differentcomposition from the second oxide semiconductor layer 144 b, the stackcan be also referred to as a heterostructure having differentcompositions.

Crystal growth of the third oxide semiconductor layer 144 c occurs underthe influence of crystallinity of the second oxide semiconductor layer144 b, so that the third oxide semiconductor layer 144 c has acrystalline structure similar to that of the second oxide semiconductorlayer 144 b. Hence, defects and states at the interface between thesecond oxide semiconductor layer 144 b and the third oxide semiconductorlayer 144 c are reduced, and thus, a highly reliable semiconductordevice can be obtained.

A state at the interface between the second oxide semiconductor layer144 b and the third oxide semiconductor layer 144 c may be mixed (oralloyed) state. The mixed state at the interface reduces damage due tostress difference between the second oxide semiconductor layer 144 b andthe third oxide semiconductor layer 144 c, leading to a reduction ofinterface scattering.

Similarly, the interface between the first oxide semiconductor layer 144a and the second oxide semiconductor layer 144 b may be alloyed.

Next, a conductive film to be the source electrode layer 142 a and thedrain electrode layer 142 b is formed over the oxide semiconductor stack144. The conductive film can be formed using a material and a methodwhich are similar to those of the gate electrode layer 110.

Note that in etching at the time of forming the source electrode layer142 a and the drain electrode layer 142 b, a region of the oxidesemiconductor stack 144 which is located between the source electrodelayer 142 a and the drain electrode layer 142 b is also etchedconcurrently and is reduced in thickness in some cases. Thus, a regionof the oxide semiconductor stack 144 which does not overlap with thesource electrode layer 142 a and the drain electrode layer 142 b has asmaller thickness than a region overlapping with the source electrodelayer 142 a and the drain electrode layer 142 b in some cases.

In the oxide semiconductor stack 144, the second oxide semiconductorlayer 144 b to be the channel formation region is sandwiched between thefirst oxide semiconductor layer 144 a and the third oxide semiconductorlayer 144 c. Hence, even when the oxide semiconductor stack 144 isetched concurrently with the etching of the source electrode layer 142 aand the drain electrode layer 142 b, the second oxide semiconductorlayer 144 b to be the channel formation region is hardly affected by theetching, and there is little possibility that the channel formationregion is etched and reduced in thickness. Thus, stable characteristicscan be exhibited.

Next, the oxide insulating layer to be used for the gate insulatinglayer 147 is formed over the source electrode layer 142 a and the drainelectrode layer 142 b. Here, the gate insulating layer 147 has atwo-layer structure in which a gate insulating layer 147 a including theoxide insulating layer and a gate insulating layer 147 b including anitride insulating layer are stacked.

The oxide insulating layer to be used for the gate insulating layer 147a can be formed using a material and a method which are similar to thoseof the insulating layer 140. In particular, a film formation conditioncausing less plasma damage may be used for forming the gate insulatinglayer 147 a in order to reduce plasma damage to the oxide semiconductorstack 144. Further, since the gate insulating layer 147 a is in contactwith the oxide semiconductor stack 144, the gate insulating layer 147 amay be formed using a film which contains oxygen in excess of thestoichiometric composition and from which oxygen is easily released byheat treatment so that oxygen can be supplied to the oxide semiconductorstack 144.

An insulating film which can be used for the gate insulating layer 147 bmay be formed using a silicon film containing oxygen and nitrogen suchas a silicon nitride oxide silicon film or a silicon oxynitride film.

Next, the gate electrode layer 148 is formed over the gate insulatinglayer 147. The gate electrode layer 148 can be formed using a materialand a method which are similar to those of the gate electrode layer 110.

The insulating layer 150 is formed over the gate electrode layer 148.The insulating layer 150 can be formed using a material and a methodwhich are similar to those of the insulating layer 140. The insulatinglayer 150 preferably contains oxygen in excess of the stoichiometriccomposition so that oxygen can be supplied to the oxide semiconductorstack 144.

Oxygen may be added to the insulating layer 150 by an ion implantationmethod, an ion doping method, a plasma immersion ion implantationmethod, or the like. By the addition of oxygen, excess oxygen can becontained in the insulating layer 150 and oxygen can be supplied fromthe insulating layer 150 to the oxide semiconductor stack 144. Notethat, in drawings, a dotted line is drawn in the insulating layer 150 toillustrate clearly that oxygen is added to the insulating layer 150 andan oxygen concentration has a peak in the insulating layer 150.

After the insulating layer 150 is formed, heat treatment is performed.The oxide semiconductor stack 144 includes oxygen vacancies formed bydamage due to plasma or etching which is performed after the oxidesemiconductor stack 144 is formed. Hence, in order to repair the damagecaused after the oxide semiconductor stack is formed, the heat treatmentis performed to supply oxygen, whereby oxygen vacancies are reduced. Thetemperature of the heat treatment is typically higher than or equal to200° C. and lower than or equal to 450° C. By the heat treatment,nitrogen in the oxide insulating layer containing nitrogen can bereleased. Note that by the heat treatment, water, hydrogen, or the likecan be released from the oxide insulating layer containing nitrogen.

For example, heat treatment is performed in a mixed atmosphere ofnitrogen and oxygen at 350° C. for one hour. By the heat treatment,hydrogen atoms and oxygen atoms in the oxide semiconductor stack 144 arereleased from the oxide semiconductor stack 144 or the interface betweenthe oxide semiconductor stack 144 and each of the insulating layers (theinsulating layer 140 and the gate insulating layer 147). In the oxidesemiconductor stack 144, sites from which oxygen atoms are releasedbecome oxygen vacancies. However, oxygen atoms contained in the oxideinsulating layer, which are in excess of the stoichiometric composition,move to the sites of the oxygen vacancies, and the oxygen vacancies arefilled with the oxygen atoms.

In this manner, nitrogen, hydrogen, or water is released from the oxidesemiconductor film by the heat treatment performed after the insulatinglayer 150 is formed, whereby the proportion of nitrogen, hydrogen, orwater in the film can be reduced to about a tenth.

The insulating layer 155 is formed over the insulating layer 150. Theinsulating layer 155 can be formed using a material and a method whichare similar to those of the insulating layer 135. The insulating layer155 can inhibit entry of an impurity from the top of the semiconductordevice into the oxide semiconductor stack 144 or can inhibit release ofoxygen in the oxide semiconductor stack 144 and the insulating layer 150to the outside from the top of the semiconductor device.

By the aforementioned process, the semiconductor device can befabricated (see FIG. 3B).

In the semiconductor device described in this embodiment, the secondoxide semiconductor layer 144 b to be the channel formation region issandwiched between the first oxide semiconductor layer 144 a and thethird oxide semiconductor layer 144 c. Thus, the channel formationregion can be apart from a surface of the oxide semiconductor stack 144,which leads to a reduction in the effect of surface scattering.

Further, the insulating layers containing oxygen in excess of thestoichiometric composition is formed so that the oxide semiconductorstack 144 is sandwiched therebetween. Hence, oxygen is supplied to theoxide semiconductor stack 144 and an oxygen vacancy in the oxidesemiconductor stack 144 is filled with the oxygen. Thus, a highlyreliable semiconductor device can be obtained.

In addition, the nitride insulating films each having a property ofblocking hydrogen or oxygen are formed so that the insulating layerscontaining excess oxygen are sandwiched therebetween. Hence, it ispossible to inhibit entry of an impurity such as hydrogen or moistureinto the oxide semiconductor stack 144 or inhibit release of oxygen fromthe oxide semiconductor layers and the insulating layers containingexcess oxygen.

Note that the above-described structure is not necessarily used for asecond transistor of the semiconductor device described in thisembodiment. For example, another mode of the semiconductor device of oneembodiment of the present invention is illustrated in each of FIGS. 4Aand 4B and FIGS. 5A to 5C. Note that in FIGS. 4A and 4B and FIGS. 5A to5C, only the second transistor is illustrated, and a first transistor,the wiring layer, and the like are omitted.

The transistor 163 illustrated in FIG. 4A is different from thetransistor 162 illustrated in FIG. 1 in that the third oxidesemiconductor layer 144 c does not cover the side surfaces of the secondoxide semiconductor layer 144 b and the side surfaces of the first oxidesemiconductor layer 144 a. The transistor 163 can be formed in such amanner that, after the first oxide semiconductor layer 144 a, the secondoxide semiconductor layer 144 b, and the third oxide semiconductor layer144 c are successively formed without exposure to the air, etching isperformed with the use of a mask such that the oxide semiconductor stack144 has an island shape. Thus, a surface of the second oxidesemiconductor layer 144 b is not exposed to the air and is not subjectedto etching treatment; hence, stable characteristics can be provided.

Further, since the third oxide semiconductor layer 144 c is etched, thegate insulating layer 147 (the gate insulating layer 147 a) and theinsulating layer 140 are in contact with each other, and thus, the oxidesemiconductor stack 144 can be surrounded by the oxide insulatinglayers. Furthermore, with the oxide insulating layers which are incontact with each other, adhesion can be improved.

In the case where the three oxide semiconductor layers are formed insuccession without exposure to the air, a manufacturing apparatus a topview of which is illustrated in FIG. 13 may be employed.

The manufacturing apparatus illustrated in FIG. 13 is single wafermulti-chamber equipment, which includes three sputtering devices 10 a,10 b, and 10 c, a substrate supply chamber 11 provided with threecassette ports 14 for holding a process substrate, load lock chambers 12a and 12 b, a transfer chamber 13, substrate heating chambers 15 and 16,and the like. Note that a transfer robot for transferring a processsubstrate is provided in each of the substrate supply chamber 11 and thetransfer chamber 13. Atmospheres of the sputtering devices 10 a, 10 b,and 10 c, the transfer chamber 13, and the substrate heating chambers 15and 16 are preferably controlled so as to hardly contain hydrogen ormoisture (i.e., so as to be an inert atmosphere, a reduced pressureatmosphere, a dry air atmosphere, or the like). For example, apreferable atmosphere is a dry nitrogen atmosphere in which the dewpoint of moisture is −40° C. or lower, preferably −50° C. or lower. Anexample of a procedure of the manufacturing steps with use of themanufacturing apparatus illustrated in FIG. 13 is as follows. A processsubstrate is transferred from the substrate supply chamber 11 to thesubstrate heating chamber 15 through the load lock chamber 12 a and thetransfer chamber 13; moisture attached to the process substrate isremoved by vacuum baking or the like in the substrate heating chamber15; the process substrate is transferred to the sputtering device 10 cthrough the transfer chamber 13; and the first oxide semiconductor layer144 a is formed in the sputtering device 10 c. Then, the processsubstrate is transferred to the sputtering device 10 a through thetransfer chamber 13 without exposure to the air, and the second oxidesemiconductor layer 144 b is formed in the sputtering device 10 a. Then,the process substrate is transferred to the sputtering device 10 bthrough the transfer chamber 13 without exposure to the air, and thethird oxide semiconductor layer 144 c is formed in the sputtering device10 b. If necessary, the process substrate is transferred to thesubstrate heating chamber 16 though the transfer chamber 13 withoutexposure to the air and subjected to heat treatment. As described above,with use of the manufacturing apparatus illustrated in FIG. 13, amanufacturing process can proceed without the process substrate beingexposed to the air. Further, with the use of the sputtering devices inthe manufacturing apparatus in FIG. 13, a process performed withoutexposure to the air can be achieved by change of the sputtering target.As the sputtering devices in the manufacturing apparatus in FIG. 13, aparallel plate sputtering device, an ion beam sputtering device, afacing-target type sputtering device, or the like may be used. In afacing-target type sputtering device, an object surface is separatedfrom plasma and thus damage in film formation is small; therefore, aCAAC-OS film having high crystallinity can be formed.

A high-purity gas in which the concentration of impurities such ashydrogen, water, a hydroxyl group, or hydride is low is used as adeposition gas for forming the oxide semiconductor layers in thesputtering devices 10 a, 10 b, and 10 c.

The heating treatment may be performed in the substrate heating chamber16 under reduced pressure, under a nitrogen atmosphere, under an oxygenatmosphere, in ultra dry air (air in which the moisture amount is lessthan or equal to 20 ppm (−55° C. by conversion into a dew point),preferably less than or equal to 1 ppm, further preferably less than orequal to 10 ppb, in the measurement with the use of a dew point meter inthe cavity ring down laser spectroscopy (CRDS) system), or under a raregas (argon, helium, or the like) atmosphere. It is preferable thatwater, hydrogen, and the like be not contained in the nitrogenatmosphere, in the oxygen atmosphere, in the ultra dry air, in the raregas atmosphere, or the like. It is also preferable that the purity ofnitrogen, oxygen, or the rare gas which is introduced into a heattreatment apparatus be set to be 6N (99.9999%) or higher, preferably 7N(99.99999%) or higher (that is, the impurity concentration is 1 ppm orlower, preferably 0.1 ppm or lower).

A transistor 164 illustrated in FIG. 4B is similar to the transistor 162in that the third oxide semiconductor layer 144 c covers the top surfaceand the side surfaces of the second oxide semiconductor layer 144 b andthe side surfaces of the first oxide semiconductor layer 144 a. However,the transistor 164 is different from the transistor 162 in that thethird oxide semiconductor layer 144 c is etched so that the edgeportions of the third oxide semiconductor layer 144 c overlap with thesource electrode layer 142 a and the drain electrode layer 142 b. Withsuch a structure, the side surfaces of the second oxide semiconductorlayer 144 b can be covered with the third oxide semiconductor layer 144c, and the insulating layer 140 can be in contact with the gateinsulating layer 147.

Further, the second transistor may have a structure including two gateelectrode layers. Transistors each including two gate electrode layersare illustrated in FIGS. 5A to 5C.

A transistor 172 illustrated in FIG. 5A has the structure in which agate electrode layer 149 is added to the transistor 162 in FIG. 1. Thegate electrode layer 149 can be formed using the same conductive film asthe wiring layer 117. By application of different potentials to the gateelectrode layer 148 and the gate electrode layer 149, the thresholdvoltage of the transistor 172 can be controlled, in a preferable manner,the negative shift in the threshold voltage can be suppressed.Alternatively, when the same potential is applied to the gate electrodelayer 148 and the gate electrode layer 149, the on-state current of thetransistor 172 can be increased.

FIG. 5B similarly illustrates a transistor 173 having the structure inwhich the gate electrode layer 149 is added to the transistor 163. FIG.5C illustrates a transistor 174 having the structure in which the gateelectrode layer 149 is added to the transistor 164.

The semiconductor device of this embodiment can be combined with any ofthe semiconductor devices of the other embodiments as appropriate.

Embodiment 2

FIG. 6A illustrates an example of a circuit diagram of a NOR circuit,which is a logic circuit, as an example of the semiconductor devicedescribed in Embodiment 1. FIG. 6B is a circuit diagram of a NANDcircuit.

In the NOR circuit illustrated in FIG. 6A, p-channel transistors 801 and802 each have a structure similar to that of the transistor 160 in FIG.1 in that a single crystal silicon substrate is used for a channelformation region, and n-channel transistors 803 and 804 each have astructure similar to structures of the transistor 162 illustrated inFIG. 1, the transistors 163 and 164 illustrated in FIGS. 4A and 4B, andthe transistors 172 to 174 illustrated in FIGS. 5A to 5C in that anoxide semiconductor film is used for a channel formation region.

In the NOR circuit illustrated in FIG. 6A, a conductive layercontrolling electrical characteristics of the transistor may be providedto overlap with a gate electrode layer with an oxide semiconductor filmprovided therebetween in each of the transistors 803 and 804. Bycontrolling the potential of the conductive layer to GND, for example,the threshold voltages of the transistors 803 and 804 are increased, sothat the transistors can be normally off.

In the NAND circuit illustrated in FIG. 6B, p-channel transistors 811and 814 each have a structure similar to that of the transistor 160 inFIG. 1, and n-channel transistors 812 and 813 each have a structuresimilar to structures of the transistor 162 illustrated in FIG. 1, thetransistors 163 and 164 illustrated in FIGS. 4A and 4B, and thetransistors 172 to 174 illustrated in FIGS. 5A to 5C in that an oxidesemiconductor film is used for a channel formation region.

In the NAND circuit illustrated in FIG. 6B, a conductive layercontrolling electrical characteristics of the transistor may be providedto overlap with a gate electrode layer with an oxide semiconductor filmprovided therebetween in each of the transistors 812 and 813. Bycontrolling the potential of the conductive layer to GND, for example,the threshold voltages of the transistors 812 and 813 are increased, sothat the transistors can be normally off.

By using a transistor including an oxide semiconductor for a channelformation region and having extremely small off-state current for thesemiconductor device in this embodiment, power consumption of thesemiconductor device can be sufficiently reduced.

A semiconductor device which is miniaturized, is highly integrated, andhas stable and excellent electrical characteristics by stackingsemiconductor elements including different semiconductor materials and amethod for manufacturing the semiconductor device can be provided.

By using the semiconductor device described in Embodiment 1, entry ofimpurities into the oxide semiconductor layer can be inhibited. Inaddition, by using the semiconductor device in which oxygen vacancies ofthe oxide semiconductor layer are reduced, a NOR circuit and a NANDcircuit which are highly reliable and exhibit stable characteristics canbe provided.

The NOR circuit and the NAND circuit including the transistor describedin Embodiment 1 are described as examples in this embodiment; however,the present invention is not limited to the circuits, and an ANDcircuit, an OR circuit, or the like can be formed using the transistordescribed in Embodiment 1.

The semiconductor device of this embodiment can be combined with any ofthe semiconductor devices of the other embodiments as appropriate.

Embodiment 3

In this embodiment, an example of a semiconductor device (memory device)which includes the semiconductor device described in Embodiment 1, whichcan hold stored data even when not powered, and which has an unlimitednumber of write cycles is described with reference to drawings.

FIG. 7A is a circuit diagram illustrating the semiconductor device ofthis embodiment.

A transistor 260 illustrated in FIG. 7A can have a structure similar tothat of the transistor 160 illustrated in FIG. 1 and easily operates athigh speed. Further, a transistor 262 can have a structure similar tostructures of the transistor 162 illustrated in FIG. 1, the transistors163 and 164 illustrated in FIGS. 4A and 4B, and the transistors 172 to174 illustrated in FIGS. 5A to 5C and enables charge to be held for along time owing to its characteristics.

Although all the transistors are n-channel transistors here, p-channeltransistors can be used as the transistors used for the semiconductordevice described in this embodiment.

In FIG. 7A, a first wiring (a 1st Line) is electrically connected to asource electrode layer of the transistor 260, and a second wiring (a 2ndLine) is electrically connected to a drain electrode layer of thetransistor 260. A third wiring (3rd Line) is electrically connected toone of a source electrode layer and a drain electrode layer of thetransistor 262, and a fourth wiring (4th Line) is electrically connectedto a gate electrode layer of the transistor 262. A gate electrode layerof the transistor 260 and the other of the source electrode layer andthe drain electrode layer of the transistor 262 are electricallyconnected to one electrode of a capacitor 264. A fifth wiring (5th Line)and the other electrode of the capacitor 264 are electrically connectedto each other.

The semiconductor device illustrate in FIG. 7A utilizes a characteristicin which the potential of the gate electrode layer of the transistor 260can be held, and thus enables data writing, holding, and reading asfollows.

Writing and holding of data will be described. First, the potential ofthe fourth wiring is set to a potential at which the transistor 262 isturned on, so that the transistor 262 is turned on. Accordingly, thepotential of the third wiring is supplied to the gate electrode layer ofthe transistor 260 and to the capacitor 264. That is, predeterminedcharge is supplied to the gate electrode layer of the transistor 260(writing). Here, charge for supplying either of two different potentiallevels (hereinafter referred to as low-level charge and high-levelcharge) is given. After that, the potential of the fourth wiring is setto a potential at which the transistor 262 is turned off, so that thetransistor 262 is turned off. Thus, the charge given to the gateelectrode layer of the transistor 260 is held (holding).

Since the off-state current of the transistor 262 is extremely low, thecharge of the gate electrode layer of the transistor 260 is held for along time.

Next, reading of data will be described. By supplying an appropriatepotential (a reading potential) to the fifth wiring while supplying apredetermined potential (a constant potential) to the first wiring, thepotential of the second wiring varies depending on the amount of chargeheld at the gate electrode layer of the transistor 260. This is becausein general, when the transistor 260 is an n-channel transistor, anapparent threshold voltage V_(th) _(—) _(H) in the case where thehigh-level charge is given to the gate electrode layer of the transistor260 is lower than an apparent threshold voltage V_(th) _(—) _(L) in thecase where the low-level charge is given to the gate electrode layer ofthe transistor 260. Here, an apparent threshold voltage refers to thepotential of the fifth line, which is needed to turn on the transistor260. Thus, the potential of the fifth wiring is set to a potential V₀which is between V_(th) _(—) _(H) and V_(th) _(—) _(L), whereby chargegiven to the gate electrode layer of the transistor 260 can bedetermined. For example, in the case where the high-level charge isgiven in writing, when the potential of the fifth wiring is set to V₀(>V_(th) _(—) _(H)), the transistor 260 is turned on. In the case wherethe low-level charge is given in writing, even when the potential of thefifth wiring is set to V₀ (<V_(th) _(—) _(L)), the transistor 260remains in an off state. Therefore, the data held can be read bymeasuring the potential of the second wiring.

Note that in the case where memory cells are arrayed to be used, onlydata of desired memory cells needs to be read. In the case where suchreading is not performed, a potential at which the transistor 260 isturned off regardless of the state of the gate electrode layer of thetransistor 260, that is, a potential smaller than V_(th) _(—) _(H) maybe given to the fifth wiring. Alternatively, a potential at which thetransistor 260 is turned on regardless of the state of the gateelectrode layer, that is, a potential higher than V_(th) _(—) _(L) maybe given to the fifth wiring.

FIG. 7B illustrates another example of one embodiment of a structure ofa memory device. FIG. 7B illustrates an example of a circuitconfiguration of a semiconductor device, and FIG. 7C is a conceptualdiagram illustrating an example of a semiconductor device. First, thesemiconductor device illustrated in FIG. 7B will be described, and then,the semiconductor device illustrated in FIG. 7C will be described.

In the semiconductor device illustrated in FIG. 7B, a bit line BL iselectrically connected to one of the source electrode layer or the drainelectrode layer of the transistor 262, a word line WL is electricallyconnected to the gate electrode layer of the transistor 262, and theother of the source electrode layer or the drain electrode layer of thetransistor 262 is electrically connected to a first terminal of acapacitor 254.

Here, the transistor 262 including an oxide semiconductor has extremelylow off-state current. For that reason, a potential of the firstterminal of the capacitor 254 (or a charge accumulated in the capacitor254) can be held for an extremely long time by turning off thetransistor 262.

Next, writing and holding of data in the semiconductor device (a memorycell 250) illustrated in FIG. 7B will be described.

First, the potential of the word line WL is set to a potential at whichthe transistor 262 is turned on, and the transistor 262 is turned on.Accordingly, the potential of the bit line BL is supplied to the firstterminal of the capacitor 254 (writing). After that, the potential ofthe word line WL is set to a potential at which the transistor 262 isturned off, so that the transistor 262 is turned off. Thus, thepotential of the first terminal of the capacitor 254 is held (holding).

Because the off-state current of the transistor 262 is extremely small,the potential of the first terminal of the capacitor 254 (or a chargeaccumulated in the capacitor) can be held for an extremely long period.

Next, reading of data will be described. When the transistor 262 isturned on, the bit line BL which is in a floating state and thecapacitor 254 are electrically connected to each other, and the chargeis redistributed between the bit line BL and the capacitor 254. As aresult, the potential of the bit line BL is changed. The amount ofchange in potential of the bit line BL varies depending on the potentialof the first terminal of the capacitor 254 (or the charge accumulated inthe capacitor 254).

For example, the potential of the bit line BL after chargeredistribution is represented by (C_(B)*V_(B0)+C*V)/(C_(B)+C), where Vis the potential of the first terminal of the capacitor 254, C is thecapacitance of the capacitor 254, C_(B) is the capacitance of the bitline BL (hereinafter also referred to as bit line capacitance), andV_(B0) is the potential of the bit line BL before the chargeredistribution. Therefore, it can be found that assuming that the memorycell 250 is in either of two states in which the potentials of the firstterminal of the capacitor 254 are V₁ and V₀ (V₁>V₀), the potential ofthe bit line BL in the case of holding the potential V₁(=(C_(B)*V_(B0)+C*V₁)/(C_(B)+C)) is higher than the potential of the bitline BL in the case of holding the potential V₀(=(C_(B)*V_(B0)+C*V₀)/(C_(B)+C)).

Then, by comparing the potential of the bit line BL with a predeterminedpotential, data can be read.

As described above, the semiconductor device illustrated in FIG. 7B canhold charge that is accumulated in the capacitor 254 for a long timebecause the amount of the off-state current of the transistor 262 isextremely small. In other words, power consumption can be adequatelyreduced because refresh operation becomes unnecessary or the frequencyof refresh operation can be extremely low. Moreover, stored data can beheld for a long time even when power is not supplied.

Next, the semiconductor device illustrated in FIG. 7C will be described.

The semiconductor device illustrated in FIG. 7C includes memory cellarrays 251 a and 251 b including a plurality of memory cells 250illustrated in FIG. 7B as memory circuits in an upper portion, and aperipheral circuit 253 in a lower portion which is necessary foroperating a memory cell array 251 (the memory cell arrays 251 a and 251b). Note that the peripheral circuit 253 is electrically connected tothe memory cell array 251.

In the structure illustrated in FIG. 7C, the peripheral circuit 253 canbe provided directly under the memory cell array 251 (the memory cellarrays 251 a and 251 b). Thus, the size of the semiconductor device canbe reduced.

It is preferable that a semiconductor material of the transistorprovided in the peripheral circuit 253 be different from that of thetransistor 262. For example, silicon, germanium, silicon germanium,silicon carbide, gallium arsenide, or the like can be used, and a singlecrystal semiconductor is preferably used. Alternatively, an organicsemiconductor material or the like may be used. A transistor includingsuch a semiconductor material can operate at sufficiently high speed.Therefore, a variety of circuits (e.g., a logic circuit or a drivercircuit) which need to operate at high speed can be favorably realizedby the transistor.

Note that FIG. 7C illustrates, as an example, the semiconductor devicein which two memory cell arrays 251 (the memory cell arrays 251 a and251 b) are stacked; however, the number of memory cell arrays to bestacked is not limited thereto. Three or more memory cell arrays may bestacked.

When a transistor including an oxide semiconductor in a channelformation region is used as the transistor 262, stored data can be heldfor a long time. In other words, power consumption can be sufficientlyreduced because a semiconductor memory device in which refresh operationis unnecessary or the frequency of refresh operation is extremely lowcan be provided.

Further, the semiconductor device described in this embodiment is thesemiconductor device described in Embodiment 1 in which the oxidesemiconductor layers are stacked to form the oxide semiconductor stackand the second oxide semiconductor layer to be the channel formationregion is apart from the surface of the oxide semiconductor stack. Thus,a highly reliable semiconductor device that exhibits stable electricalcharacteristics can be obtained.

Embodiment 4

In this embodiment, examples of application of the semiconductor devicedescribed in any of the above embodiments to electronic devices such asa mobile phone, a smartphone, or an electronic book will be describedwith reference to FIG. 8, FIG. 9, FIG. 10, and FIGS. 11A and 11B.

FIG. 8 is a block diagram of an electronic device. An electronic deviceillustrated in FIG. 8 includes an RF circuit 901, an analog basebandcircuit 902, a digital baseband circuit 903, a battery 904, a powersupply circuit 905, an application processor 906, a flash memory 910, adisplay controller 911, a memory circuit 912, a display 913, a touchsensor 919, an audio circuit 917, a keyboard 918, and the like. Thedisplay 913 includes a display portion 914, a source driver 915, and agate driver 916. The application processor 906 includes a CPU 907, a DSP908, and an interface (IF) 909. In general, the memory circuit 912includes an SRAM or a DRAM; by employing any of the semiconductordevices described in the above embodiments for the memory circuit 912,writing and reading of data can be performed at high speed, data can beheld for a long time, and power consumption can be sufficiently reduced.

FIG. 9 illustrates an example in which any of the semiconductor devicesdescribed in the above embodiments is used for a memory circuit 950 in adisplay. The memory circuit 950 illustrated in FIG. 9 includes a memory952, a memory 953, a switch 954, a switch 955, and a memory controller951. Further, the memory circuit is connected to a display controller956 which reads and controls image data input through a signal line(input image data) and data stored in the memories 952 and 953 (storedimage data), and is also connected to a display 957 which displays animage based on a signal input from the display controller 956.

First, image data (input image data A) is formed by an applicationprocessor (not illustrated). The input image data A is stored in thememory 952 though the switch 954. The image data (stored image data A)stored in the memory 952 is transmitted to the display 957 through theswitch 955 and the display controller 956, and is displayed on thedisplay 957.

In the case where the input image data A is not changed, the storedimage data A is read from the memory 952 through the switch 955 by thedisplay controller 956 normally at a frequency of approximately 30 Hz to60 Hz.

Next, for example, when a user performs an operation to rewrite a screen(i.e., when the input image data A is changed), the applicationprocessor produces new image data (input image data B). The input imagedata B is stored in the memory 953 through the switch 954. Also duringthis time, the stored image data A is regularly read from the memory 952through the switch 955. After the completion of storing the new imagedata (stored image data B) in the memory 953, from the next frame forthe display 957, the stored image data B starts to be read, istransmitted to the display 957 through the switch 955 and the displaycontroller 956, and is displayed on the display 957. This readingoperation continues until another new image data is stored in the memory952.

By alternately writing and reading image data to and from the memory 952and the memory 953 as described above, images are displayed on thedisplay 957. Note that the memory 952 and the memory 953 are notnecessarily separate memories and a single memory may be divided andused. By employing any of the semiconductor devices described in theabove embodiments for the memory 952 and the memory 953, data can bewritten and read at high speed and held for a long time, and powerconsumption can be sufficiently reduced. Further, a semiconductor devicewhich is hardly affected by entry of water, moisture, and the like fromthe outside and has high reliability can be provided.

FIG. 10 is a block diagram of an electronic book. The electronic book inFIG. 10 includes a battery 1001, a power supply circuit 1002, amicroprocessor 1003, a flash memory 1004, an audio circuit 1005, akeyboard 1006, a memory circuit 1007, a touch panel 1008, a display1009, and a display controller 1010.

Here, the semiconductor device described in any of the above embodimentscan be used for the memory circuit 1007 in FIG. 10. The memory circuit1007 has a function of temporarily storing the contents of a book. Forexample, when a user uses a highlight function, the memory circuit 1007stores and holds data of a portion specified by the user. Note that thehighlight function is used to make a difference between a specificportion and the other portions while reading an electronic book, bymarking the specific portion, e.g., by changing the display color,underlining, making characters bold, changing the font of characters, orthe like. In order to store the data for a short time, the data may bestored in the memory circuit 1007. In order to store the data for a longtime, the data stored in the memory circuit 1007 may be copied to theflash memory 1004. Also in such a case, by employing the semiconductordevices described in any of the above embodiments, data can be writtenand read at high speed and held for a long time, and power consumptioncan be sufficiently reduced. Further, a semiconductor device which ishardly affected by entry of water, moisture, and the like from theoutside and which has high reliability can be provided.

FIGS. 11A and 11B illustrate a specific example of an electronic device.FIGS. 11A and 11B illustrate a foldable tablet terminal The tabletterminal is opened in FIG. 11A. The tablet terminal includes a housing9630, a display portion 9631 a, a display portion 9631 b, a display modeswitch 9034, a power switch 9035, a power saver switch 9036, a clasp9033, and an operation switch 9038.

The semiconductor device described in Embodiment 1 can be used for thedisplay portion 9631 a and the display portion 9631 b, so that thetablet terminal can have high reliability. In addition, the memorydevice described in the above embodiment may be applied to thesemiconductor device of this embodiment.

Part of the display portion 9631 a can be a touch panel region 9632 aand data can be input when a displayed operation key 9638 is touched.Although a structure in which a half region in the display portion 9631a has only a display function and the other half region also has a touchpanel function is shown as an example, the display portion 9631 a is notlimited to the structure. For example, the display portion 9631 a candisplay keyboard buttons in the whole region to be a touch panel, andthe display portion 9631 b can be used as a display screen.

As in the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region 9632 b. When a keyboard display switching button9639 displayed on the touch panel is touched with a finger, a stylus, orthe like, a keyboard can be displayed on the display portion 9631 b.

Touch input can be performed in the touch panel region 9632 a and thetouch panel region 9632 b at the same time.

The display mode switch 9034 can switch the display between portraitmode, landscape mode, and the like, and between monochrome display andcolor display, for example. The power saver switch 9036 can controldisplay luminance in accordance with the amount of external light in useof the tablet terminal detected by an optical sensor incorporated in thetablet terminal. In addition to the optical sensor, another detectiondevice including a sensor for detecting inclination, such as a gyroscopeor an acceleration sensor, may be incorporated in the tablet terminal.

Although the display portion 9631 a and the display portion 9631 b havethe same display area in FIG. 11A, one embodiment of the presentinvention is not limited to this structure. The display portion 9631 aand the display portion 9631 b may have different areas or differentdisplay quality. For example, one of them may be a display panel thatcan display higher-definition images than the other.

The tablet terminal is closed in FIG. 11B. The tablet terminal includesthe housing 9630, a solar cell 9633, a charge and discharge controlcircuit 9634, a battery 9635, and a DCDC converter 9636. In FIG. 11B, astructure including the battery 9635 and the DCDC converter 9636 isillustrated as an example of the charge and discharge control circuit9634.

Since the tablet terminal is foldable, the housing 9630 can be closedwhen the tablet terminal is not used. As a result, the display portion9631 a and the display portion 9631 b can be protected; thus, a tabletterminal which has excellent durability and excellent reliability interms of long-term use can be provided.

In addition, the tablet terminal illustrated in FIGS. 11A and 11B canhave a function of displaying a variety of kinds of data (e.g., a stillimage, a moving image, and a text image), a function of displaying acalendar, a date, the time, or the like on the display portion, atouch-input function of operating or editing the data displayed on thedisplay portion by touch input, a function of controlling processing bya variety of kinds of software (programs), and the like.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

EXPLANATION OF REFERENCE

100: substrate, 102: element isolation insulating layer, 104: insulatinglayer, 108: gate insulating layer, 110: gate electrode layer, 112:wiring layer, 114: wiring layer, 115: wiring layer, 115 a: wiring layer,115 b: wiring layer, 115 c: wiring layer, 116: wiring layer, 117: wiringlayer, 120: insulating layer, 135: insulating layer, 140: insulatinglayer, 142 a: source electrode layer, 142 b: drain electrode layer, 144:oxide semiconductor stack, 144 a: oxide semiconductor layer, 144 b:oxide semiconductor layer, 144 c: oxide semiconductor layer, 147: gateinsulating layer, 147 a: gate insulating layer, 147 b: gate insulatinglayer, 148: gate electrode layer, 149: gate electrode layer, 150:insulating layer, 155: insulating layer, 160: transistor, 162:transistor, 163: transistor, 164: transistor, 172: transistor, 173:transistor, 174: transistor, 250: memory cell, 251: memory cell array,251 a: memory cell array, 251 b: memory cell array, 253: peripheralcircuit, 254: capacitor, 260: transistor, 262: transistor, 264:capacitor, 801: transistor, 802: transistor, 803: transistor, 804:transistor, 811: transistor, 812: transistor, 813: transistor, 814:transistor, 901: RF circuit, 902: analog baseband circuit, 903: digitalbaseband circuit, 904: battery, 905: power supply circuit, 906:application processor, 907: CPU, 908: DSP, 909: interface, 910: flashmemory, 911: display controller, 912: memory circuit, 913: display, 914:display portion, 915: source driver, 916: gate driver, 917: audiocircuit, 918: keyboard, 919: touch sensor, 950: memory circuit, 951:memory controller, 952: memory, 953: memory, 954: switch, 955: switch,956: display controller, 957: display, 1001: battery, 1002: power supplycircuit, 1003: microprocessor, 1004: flash memory, 1005: audio circuit,1006: keyboard, 1007: memory circuit, 1008: touch panel, 1009: display,1010: display controller, 9033: clasp, 9034: switch, 9035: power switch,9036: switch, 9038: operation switch, 9630: housing, 9631 a: displayportion, 9631 b: display portion, 9632 a: region, 9632 b: region, 9633:solar cell, 9634: charge and discharge control circuit, 9635: battery,9636: DCDC converter, 9638: operation key, 9639: button

This application is based on Japanese Patent Application serial no.2012-178634 filed with Japan Patent Office on Aug. 10, 2012, the entirecontents of which are hereby incorporated by reference.

1. A semiconductor device comprising: a first oxide semiconductor layerover an insulating surface; a second oxide semiconductor layer over thefirst oxide semiconductor layer; a third oxide semiconductor layer overthe second oxide semiconductor layer; a first insulating layer over thethird oxide semiconductor layer; and a first gate electrode over thefirst insulating layer, the first gate electrode overlapping with thefirst oxide semiconductor layer, the second oxide semiconductor layer,and the third oxide semiconductor layer, wherein each of the secondoxide semiconductor layer and the third oxide semiconductor layer has acrystalline structure.
 2. The semiconductor device according to claim 1,further comprising: a source electrode and a drain electrode over andelectrically connected to the first oxide semiconductor layer, thesecond oxide semiconductor layer, and the third oxide semiconductorlayer.
 3. The semiconductor device according to claim 1, furthercomprising: a second insulating layer having the insulating surface; anda second gate electrode under the second insulating layer, the secondgate electrode overlapping with the first oxide semiconductor layer, thesecond oxide semiconductor layer, and the third oxide semiconductorlayer.
 4. The semiconductor device according to claim 1, wherein thefirst oxide semiconductor layer has an amorphous structure.
 5. Thesemiconductor device according to claim 1, wherein each of the firstoxide semiconductor layer, the second oxide semiconductor layer, and thethird oxide semiconductor layer comprises at least one of indium, zinc,and gallium.
 6. The semiconductor device according to claim 1, whereineach of the first oxide semiconductor layer, the second oxidesemiconductor layer, and the third oxide semiconductor layer comprisesindium, and wherein a proportion of indium in the second oxidesemiconductor layer is higher than a proportion of indium in the firstoxide semiconductor layer and a proportion of indium in the third oxidesemiconductor layer.
 7. The semiconductor device according to claim 1,wherein the first insulating layer contains oxygen in excess of astoichiometric composition.
 8. The semiconductor device according toclaim 1, wherein a concentration of silicon or carbon in each of thefirst oxide semiconductor layer and the third oxide semiconductor layeris lower than or equal to 3×10¹⁸ atoms/cm³.
 9. A semiconductor devicecomprising: a first insulating layer over a substrate, the firstinsulating layer comprising aluminum and oxygen; a first gate electrodeover the first insulating layer; a second insulating layer over thefirst gate electrode; a first oxide semiconductor layer over the secondinsulating layer, the first oxide semiconductor layer overlapping withthe first gate electrode; a second oxide semiconductor layer over thefirst oxide semiconductor layer, the second oxide semiconductor layeroverlapping with the first gate electrode; a source electrode and adrain electrode over and electrically connected to the first oxidesemiconductor layer and the second oxide semiconductor layer; a thirdinsulating layer over the second oxide semiconductor layer, the sourceelectrode, and the drain electrode; and a second gate electrode over thethird insulating layer, the second gate electrode overlapping with thefirst oxide semiconductor layer and the second oxide semiconductorlayer.
 10. The semiconductor device according to claim 9, wherein eachof the first oxide semiconductor layer and the second oxidesemiconductor layer has a crystalline structure.
 11. The semiconductordevice according to claim 9, wherein each of the first oxidesemiconductor layer and the second oxide semiconductor layer comprisesat least one of indium, zinc, and gallium.
 12. The semiconductor deviceaccording to claim 9, wherein each of the first oxide semiconductorlayer and the second oxide semiconductor layer comprises indium.
 13. Thesemiconductor device according to claim 9, wherein the second insulatinglayer contains oxygen in excess of a stoichiometric composition.
 14. Thesemiconductor device according to claim 9, wherein a concentration ofsilicon or carbon in the first oxide semiconductor layer is lower thanor equal to 3×10¹⁸ atoms/cm³.
 15. A semiconductor device comprising: afirst insulating layer over a substrate, the first insulating layercomprising aluminum and oxygen; a first gate electrode over the firstinsulating layer; a second insulating layer over the first gateelectrode; a first oxide semiconductor layer over the second insulatinglayer, the first oxide semiconductor layer overlapping with the firstgate electrode; a second oxide semiconductor layer over the first oxidesemiconductor layer, the second oxide semiconductor layer overlappingwith the first gate electrode; a third oxide semiconductor layer overthe second oxide semiconductor layer, the third oxide semiconductorlayer overlapping with the first gate electrode; a source electrode anda drain electrode over and electrically connected to the first oxidesemiconductor layer, the second oxide semiconductor layer, and the thirdoxide semiconductor layer; a third insulating layer over the third oxidesemiconductor layer, the source electrode, and the drain electrode; anda second gate electrode over the third insulating layer, the second gateelectrode overlapping with the first oxide semiconductor layer, thesecond oxide semiconductor layer, and the third oxide semiconductorlayer.
 16. The semiconductor device according to claim 15, wherein thefirst oxide semiconductor layer has an amorphous structure, and whereineach of the second oxide semiconductor layer and the third oxidesemiconductor layer has a crystalline structure.
 17. The semiconductordevice according to claim 15, wherein each of the first oxidesemiconductor layer, the second oxide semiconductor layer, and the thirdoxide semiconductor layer comprises at least one of indium, zinc, andgallium.
 18. The semiconductor device according to claim 15, whereineach of the first oxide semiconductor layer, the second oxidesemiconductor layer, and the third oxide semiconductor layer comprisesindium, and wherein a proportion of indium in the second oxidesemiconductor layer is higher than a proportion of indium in the firstoxide semiconductor layer and a proportion of indium in the third oxidesemiconductor layer.
 19. The semiconductor device according to claim 15,wherein the second insulating layer contains oxygen in excess of astoichiometric composition.
 20. The semiconductor device according toclaim 15, wherein a concentration of silicon or carbon in each of thefirst oxide semiconductor layer and the third oxide semiconductor layeris lower than or equal to 3×10¹⁸ atoms/cm³.